1. Field of the Invention
The present invention generally relates to ghost cancelling circuits and, more particularly, is directed to a circuit for detecting waveform distortion of a video signal used in a ghost eliminating or cancelling circuit.
2. Description of the Prior Art
In the television receivers, cancellation or removal of a ghost wave component of a received video signal can be performed in the following manner.
That is, a reference signal for ghost cancellation (hereinafter referred to simply as a GCR signal) is added to the video signal in a transmitter side.
In a receiver side, a waveform of the GCR signal (including a ghost wave component) of the received video signal is compared with a waveform of a GCR signal formed in the receiver side to extract the ghost wave component and to control the transmission characteristic of a transversal filter, for example, so as to eliminate the extracted ghost wave component.
As the GCR signal, such a signal SGCR shown in FIG. 1A has been used.
In FIG. 1A, a symbol HD represents a horizontal synchronizing pulse and BRST a burst signal. As shown in FIG. 1A, a first GCR signal is formed to have a bar pattern waveform which is disposed succeeding to a horizontal period and has a width of 44.7 .mu. sec. and a level of 70 IRE. The rising characteristics of the bar pattern waveform is a ringing characteristics of sin X/X.
Further, a second GCR signal PDS is formed to have a pedestal waveform whose flat-topped level is 0 as shown in FIG. 1B.
As shown in FIG. 2A, the GCR signal is constituted with a repetition period of 8 fields of the video signal in a manner that the first GCR signal GCR is inserted in a 18'th or a 281'th line of each of the 1st, 3rd, 6th and 8th fields while the second GCR signal PDS is inserted in a 18'th or a 281'th line of each of the remaining 2nd, 4th, 5th and 7th fields and the thus constructed GCR signal SGCR is inserted in the video signal and transmitted.
Supposing that the first to eighth GCR signals SGCR are called signals S1-S8, respectively, if these signals S1 to S8 are operated in the receiver side based on the equation of FIG. 2B, the result of the operation will be a signal GCR. If the ghost wave component is included in the received GCR signal, the result of the operation also includes a ghost wave component Sg of the signal GCR.
Thus, it is possible to eliminate the ghost component on the basis of the signal GCR (and the component Sg) of the operation result.
In this case, each of the burst signal BRST, chrominance signals and the horizontal synchronizing pulses HD exhibits the same phase every eight fields, so that each of the burst signal BRST, chrominance signal and horizontal synchronizing pulse HD is cancelled when performing the operations of the signals S1-S8.
Thus, the signal GCR (and the ghost wave component Sg) obtained through the operation does not include any of the burst signal BRST, chrominance signal and the horizontal synchronizing pulse HD, so that the elimination or cancellation of a so-called front ghost and rear ghost and waveform equalization can be performed within a range of 45 .mu. sec. at maximum. Further, an erroneous detection can be prevented for a long ghost having a width of about 80 .mu. sec.
FIG. 3 is a block diagram showing an example of the conventional ghost cancelling circuit using the GCR signal SGCR In FIG. 3, a composite color video signal SY added with the above-described GCR signal SGCR is picked up from a video signal detecting circuit 1 and applied to an analog-to-digital (A/D) converter 2 to be converted into a digital video signal SY of 8 bits for one sample, for example. The digital video signal SY is then applied to a digital-to-analog (D/A) converter 4 through a transversal filter 3 with 640 steps or taps, for example, to be converted into an original analog video signal SY and then taken out from a terminal 5.
In that time, in a detecting circuit 10, a ghost wave component is detected from the GCR signal SGCR to control the transmission characteristic of the transversal filter 3 on the basis of the detected output to thereby eliminate the ghost wave component from the video signal.
The operation shown in FIG. 2B can be rewritten into the equation of FIG. 2C which means that the operation can be performed by sequentially accumulating the GCR signals SGCR in the respective fields.
Thus, in the detecting circuit 10, the digitized video signal SY from the filter 3 is applied to a gate circuit 11 to extract the GCR signal SGCR (including foregoing and succeeding detection periods), and then the extracted signal SGCR is applied to a buffer memory 12 to hold at every one field period, all of the GCR signals SGCR during the period.
The GCR signals SGCR stored in the memory 12 are applied to an operation circuit 21. This operation circuit 21 and the following circuits 22 to 25 are constituted by a microcomputer 20 and software in practice but in this figure they are represented by hardware.
In the operation circuit 21, the GCR signals SGCR stored in the memory 12 are sequentially read out and added or subtracted according to the equation of FIG. 2C at every one field period to thereby obtain the signal GCR and the ghost wave component Sg as the result of the operation of the eight fields. The signal GCR and the component Sg thus obtained are applied to the subtracting circuit 22, to which a reference GCR signal 23 with a reference waveform of the signal GCR shown in FIG. 1A generated by a reference GCR signal forming circuit 23 is also applied.
The subtracting circuit 22 performs the subtraction operation between these signals to thereby deliver the ghost wave component Sg of the received signal GCR. Now, this ghost wave component Sg also represents an error component resulting from the fact that the ghost wave component can not be cancelled completely from the received video signal.
The transversal filter has a pulse response characteristic but the signal GCR has a bar pattern waveform, and so the ghost wave component Sg delivered from the subtracting circuit 22 is applied to the differentiating circuit 24 to be converted into a differentiated pulse Pg which is in turn applied to the converting circuit 25.
The pulse Pg is converted in the converting circuit 25 into a signal ST representing a correction or modification value of a tap coefficient or a tap gain of the transversal filter 3 and then applied to the filter 3 to thereby control the transmission characteristic thereof to a direction of cancelling and eliminating the ghost wave component Sg of the GCR signal SGCR delivered from the filter 3.
This operation is repeatedly performed to gradually adjust the characteristic of the transversal filter 3 to thereby gradually converge the characteristic thereof to that for eliminating the ghost wave component Sg of the GCR signal SGCR.
When the characteristic of the filter 3 is converged sufficiently, the ghost wave component Sg of the GCR signal SGCR delivered from the filter 3 will be decreased to a negligible small level and also the ghost component of the inherent video signal SY is decreased to a negligible small level by the filter 3.
Accordingly, the video signal SY from which the ghost wave component is cancelled can be taken out from the terminal 5.
The above-described conventional ghost cancelling circuit is described in "Ghost Cancel Reference Signal System", the Journal of National Conference held by the Institute of Television Engineers of Japan, 1989.
In this conventional circuit, the characteristic of the transversal filter 3 is gradually adjusted to thereby converge it to a required characteristic.
In the system having an adaptive function as described above, as an algorithm for improving an evaluation value by repeatedly adjusting the parameter of the system, the hill climbing method is well known. According to the ADA (active division algorithm) methods as one of the hill climbing method, a value Cj(v) of a parameter at a .gamma.'th process is modified according to the following equation. ##EQU1## where D represents the evaluation function, Cj the parameter (j= 1 .about. n), Cj(.gamma.) the value of a parameter at a .gamma.'th process, .alpha. the coefficient, and .DELTA.C the value to be modified.
This ADA method can be applied to the setting or adjustment of the tap coefficient of the transversal filter 3 and in this case the value .DELTA.C of the above-described equation will be the correction or modification value of the tap coefficient of the signal ST.
However, if the tap coefficient of the transversal filter 3 is set according to the ADA method, it takes as long a time as about 5 seconds depending on the processing ability of the microcomputer 20 to converge the characteristic of the transversal filter to the required characteristic.
If it requires such a long time as 5 seconds to converge the characteristic, the GCR signal SGCR delivered from the filter may include a noise component if a noise signal is received at the receiver side during a converging period of the characteristic. If a noise signal is included in the GCR signal SGCR, the converging operation in the filter is distorted to thereby delay the convergence thereof.